branchmap: fix blank line position

The blank line was after was after the `if` condition instead of before.

mercurial/branchmap.py

@@ -210,8 +210,8 b' class branchcache(dict):'
             if not nodes:
                 droppednodes.extend(nodes)
                 del self[branch]
+
         if ((not self.validfor(repo)) or (self.tipnode in droppednodes)):
-
             # cache key are not valid anymore
             self.tipnode = nullid
             self.tiprev = nullrev